There are presently many different types of plate and fixture systems for securing two or more bones or bone fragments in relative position so that the bones may fuse or heal, or so that tissue adjacent the bones may heal without disruption from the movement of the secured bones. As used herein, the term bone may refer to a bone, or a bone fragment or portion, and the term may refer to a portion of a bone that is covered with another material, such as the endplates covering the top and bottom surface of a vertebra. Also as used herein, the term fusion refers to the joining of materials, such as bone or graft material, and the fusion site is the entire region in which fusion may be desired. These systems have been used to secure spinal vertebrae such as cervical vertebrae.
Bone plate systems are typically used to assist or direct spinal fusion or vertebral healing procedures. These procedures promote earlier post-operative patient mobility and improve success in correcting spinal deformities while decreasing the need for post-operative collars and the incidence of graft dislodgement if a graft is used.
Furthermore, these systems have been found to assist in controlling and/or exerting a compressive loading force applied to the surgical site. By applying a compressive load, it has been found that bone heals more optimally and with greater integrity, a principle known as Wolff's Law.
Some prior bone plate systems seek to provide a compressive force while allowing the vertebrae to settle naturally under the force of gravity by offering bone anchors such as screws or alternatively coupling members that couple the screw heads to the bone plates that can pivot with respect to the plate as the vertebrae shift, settle, and/or curvature of the spine is altered. Many previous bone plate systems do not even allow such motion, and many that do provide inadequate control over the manner in which the vertebrae settle under compression. Additionally, if this shifting or settling of vertebrae is improperly or inadequately accounted for, additional stress may be added to the vertebrae and an undesirable load path through the spine may be created, hindering the recovery, grafting, and/or fusion process.
Another manner for permitting compressive loads between joined bones is to utilize a dynamic plate having at least one elongated screw aperture that allows settling of the vertebrae by gravity by allowing at least one secured bone and its associated bone anchor to move relative to the plate. However, heretofore known arrangements of standard and dynamized apertures in such plates provide less than optimal capacity for controlling the movement and/or compression between more than two tiers of secured vertebrae and/or many previously known bone plates did not provide sufficient movement to allow the spine to settle naturally as a portion of the spine is compressed during the recovery period. Inasmuch as these prior bone plate systems allowed for some settling of the spine, this settling would cause the spine to be inclined to exhibit an altered degree of curvature, which prior dynamic bone plate systems failed to accommodate. If the spine is not allowed to adapt to this different degree of curvature and thus reach a more stable configuration, an undesirable or improper load path through the spine may be created, hindering the recovery, grafting, and/or fusion process.
Another shortcoming of many bone plate systems is the backing out or loosening of the bone anchors, which are often bone screws. If the bone anchors loosen and/or back out, the bones are not properly secured and may be allowed to move relative to one another in an uncontrolled manner. This may compromise the ability to achieve optimal bone fusion and bone alignment, and it may lead to loss of graft material and damage or loss of bone. Furthermore, when the plate is a dynamic or dynamized plate, such that at least some bone anchors may be permitted to move relative to the plate, these issues may be further compounded or exacerbated by a bone anchor backing out. Additionally, in the case of anterior cervical plates, a bone anchor backing out could hinder swallowing and cause irritation or even a puncture wound to the esophagus, which may lead to infection or even death.
Some known bone plate systems offer resilient base members such as c-clips which house at least a portion of the head of the bone anchor. Often, the base member will be compressed upon entering a throughbore and allowed to expand at least partially to its original shape. These resilient base members, however, have clear drawbacks. First, under certain conditions, many resilient base members may be recompressed and pushed out of the throughbore, which may allow the base member and/or bone anchor to loosen and back out of the bone plate. Furthermore, resilient base members often expand only partially to their original configuration, thus imposing stress to the inner walls of the throughbore, causing increased friction and thus hindering the bone anchor from accommodating spinal shifts, compression, and/or changes in curvature.
Accordingly, there is a need for improved bone plates, bone plate systems, mechanisms to inhibit bone anchor back-out, and improved tools and methods for utilizing bone plate systems.